diff --git a/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java
new file mode 100644
index 000000000..e9620c9bd
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java
@@ -0,0 +1,45 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.concurrent.TimeUnit.MINUTES;
+
+@ThreadSafe
+@NotNullByDefault
+interface TorReachabilityMonitor {
+
+	/**
+	 * How long the Tor plugin needs to be continuously
+	 * {@link Plugin.State#ACTIVE active} before we assume our contacts can
+	 * reach our hidden service.
+	 */
+	long REACHABILITY_PERIOD_MS = MINUTES.toMillis(10);
+
+	/**
+	 * Starts the monitor.
+	 */
+	void start();
+
+	/**
+	 * Destroys the monitor.
+	 */
+	void destroy();
+
+	/**
+	 * Adds an observer that will be called when our Tor hidden service becomes
+	 * reachable. If our hidden service is already reachable, the observer is
+	 * called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the monitor is
+	 * {@link #destroy() destroyed}.
+	 */
+	void addObserver(TorReachabilityObserver o);
+
+	interface TorReachabilityObserver {
+
+		void onTorReachable();
+	}
+}
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java
new file mode 100644
index 000000000..a82d1a966
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java
@@ -0,0 +1,128 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.TorConstants.ID;
+
+@ThreadSafe
+@NotNullByDefault
+class TorReachabilityMonitorImpl
+		implements TorReachabilityMonitor, EventListener {
+
+	private final Executor ioExecutor;
+	private final TaskScheduler taskScheduler;
+	private final PluginManager pluginManager;
+	private final EventBus eventBus;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private boolean reachable = false, destroyed = false;
+
+	@GuardedBy("lock")
+	private final List<TorReachabilityObserver> observers = new ArrayList<>();
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable task = null;
+
+	@Inject
+	TorReachabilityMonitorImpl(
+			@IoExecutor Executor ioExecutor,
+			TaskScheduler taskScheduler,
+			PluginManager pluginManager,
+			EventBus eventBus) {
+		this.ioExecutor = ioExecutor;
+		this.taskScheduler = taskScheduler;
+		this.pluginManager = pluginManager;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public void start() {
+		eventBus.addListener(this);
+		Plugin plugin = pluginManager.getPlugin(ID);
+		if (plugin != null && plugin.getState() == ACTIVE) onTorActive();
+	}
+
+	@Override
+	public void destroy() {
+		eventBus.removeListener(this);
+		synchronized (lock) {
+			destroyed = true;
+			onTorInactive();
+			observers.clear();
+		}
+	}
+
+	@Override
+	public void addObserver(TorReachabilityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (reachable) callNow = true;
+			else observers.add(o);
+		}
+		if (callNow) o.onTorReachable();
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorActive();
+		} else if (e instanceof TransportInactiveEvent) {
+			TransportInactiveEvent t = (TransportInactiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorInactive();
+		}
+	}
+
+	private void onTorActive() {
+		synchronized (lock) {
+			if (destroyed || task != null) return;
+			task = taskScheduler.schedule(this::onTorReachable, ioExecutor,
+					REACHABILITY_PERIOD_MS, MILLISECONDS);
+		}
+	}
+
+	private void onTorInactive() {
+		synchronized (lock) {
+			reachable = false;
+			if (task != null) task.cancel();
+			task = null;
+		}
+	}
+
+	@IoExecutor
+	private void onTorReachable() {
+		List<TorReachabilityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			reachable = true;
+			observers = new ArrayList<>(this.observers);
+			this.observers.clear();
+			task = null;
+		}
+		for (TorReachabilityObserver o : observers) o.onTorReachable();
+	}
+}